Whisk together the marinade ingredients (excluding the butter) in a small mixing bowl.
Place the flank steak in a gallon-sized zippered bag and add in the marinade.
Marinate in the fridge for at least an hour or up to overnight.
When you are ready to cook the flank steak, gently pat it dry before adding it to a piping hot skillet with a tablespoon of butter. (add the butter after the skillet heats up)
To cook on medium-high heat, it will take about 10 minutes total to cook, flipping once. Keep an eye on it while it cooks, and if necessary, reduce the heat to medium to ensure the sides don't burn.
Once the steak's desired internal temperature has been reached, remove it from the heat to rest for 5-10 minutes before cutting and serving.
Notes
Bring the steaks to room temperature - It's important to let the flank steak come to room temperature before making it to ensure even cooking.
Pat your steak dry - To achieve that crispy, flavorful crust; it's essential the surface of the meat is dry.
Give the meat some space after cooking - Letting the steak rest before slicing will allow the juices to redistribute throughout the meat, resulting in a more tender and juicy steak.
Flank steak lasts up to 4 days in an airtight container in the fridge.